Consider a tricycle.  Start with the trike 
at rest, with one pedal straight up and the 
other straight down.  If you push forward on 
the down pedal, which way will the trike go?

   Here is a nice statement of this problem
with a couple of possible arguments for
which way it should go.

   Jim Mandes and Sterling Thomas from our
Spring 2003 121 class tried it out.  Here 
are videos of their results, along with 
their explanation.

Thanks Jim and Sterling.